The manufacture of ethanol from cellulose-containing materials has been known for many years. Two reactions are needed. The first is the hydrolysis of cellulose into fermentable sugars, and this reaction is also referred to as "saccharification". The second reaction involves the conversion of the sugars to ethanol, and this is commonly done by a fermentation with yeast.
The saccharification can be accomplished by addition of acids, as described in the U.S. Pat. Nos. of Moore, 1,323,540, and in the patent of Church et al, 4,201,596. The saccharification can also be accomplished by cellulases, as described in the patent of Mandels et al, 3,764,475, and in the patent of Ghose, 3,642,580. Additional agents for microbiological saccharification include thermophilic cellulolytic sporocytophaga, as described in the patent of Bellamy, 4,094,742.
The formation of ethanol from the sugars can be accomplished by yeasts such as Saccharomyces cerevisiae, described in the patent of Griesbach, 2,802,774, and Fusarium oxysporum, as referred to in the article by Sitton et al on page 52 of the December 1979 issue of Chemical Engineering Progress. Other useful microorganisms are the ethanol-producing bacillus described by Bellamy in U.S. Pat. No. 4,094,742. Additional promising microorganisms for either the saccharification or fermentation are listed by Flickinger, Biotechnology and Bioengineering, Vol. XXI, Suppl. 1, 27-48 (1980).
The commercial manufacture of ethanol from cellulose has lagged because of several major process problems involving saccharification, the reactivity of the raw materials and the energy required to purify the ethanol.
Saccharification has been a problem because acid saccharification is troubled by corrosion problems, by problems in recovering the acid, and from the formation of non-fermentable compounds from the high-temperature reaction between the acid and the sugars. Saccharification by enzyme has been uneconomical because of the slow reaction of the enzyme, by the high cost of the enzyme and by the inhibiting effect of the sugars on the reaction rate.
Reactivity of the cellulose-containing raw materials has been a problem. In practice, it is usually possible to only get about 20 pounds of ethanol from 100 pounds of dry wood or the like cellulosic material. The unreactive fraction of wood, for example, is usually over 50% of the initial weight. The ethanol yields can be greatly increased by starting with a purified cellulose or a precipitated amorphous cellulose. The use of such treated types of cellulose causes the resulting ethanol to be uneconomical for general industrial use.
A third problem is that sugar solutions from saccharification of cellulose tend to be dilute solutions with glucose content of 5% or less, and the fermentation of such solutions leads to an ethanol concentration of about 2.4%, which is too dilute for economical purification in a distillation column. Economic distillation normally requires an incoming ethanol concentration of at least 5% or more. The cellulose concentration in the saccharification reaction is limited to concentrations which can be readily stirred, and this is typically in the range of 6 to 10% solids, although high-solids acid hydrolysis is under development. The normal corrective action is to concentrate the glucose solution by evaporation prior to the fermentation step, but this consumes unacceptable quantities of energy.
There have been many proposals to eliminate the various problems, such as aforementioned. U.S. Pat. Nos. 3,990,944, 4,094,742 and my earlier issued patent, 4,009,075, describe the use of simultaneous saccharification and fermentation for the purpose of removing the inhibiting sugars as they are formed by continually converting them to ethanol. Ethanol is not a serious inhibitor to saccharification, and satisfactory enzymatic saccharification in the presence of ethanol concentrations up to 10% has been reported by Blotkamp et al, "Enzymatic Hydrolysis of Cellulose and Simultaneous Fermentation to Alcohol", A.I.Ch.E. Symp. Series Vol. 74, No. 181, 85-90, (1978). In the process for the enzymatic conversion of cellulosic materials to sugar, as described in the patent of Wilke et al, 3,972,775, a saccharification system is disclosed in which sugars are washed from withdrawn unreacted material and the unreacted material is subsequently returned to the saccharification vessel for more reaction, while additional new cellulose is continuously added to the saccharification vessel. The process produces sugars rather than ethanol. Moreover, no recycling of ethanol-containing liquors from the fermentation reaction back into the saccharification reaction is provided. This is an important step of my present process, and which results in obtaining an increased ethanol concentration in the liquid being sent to the beer still. This increase in ethanol concentration is not achieved without the recycling treatment described.
The process of my invention produces ethanol at a lower cost than heretofore, and provides a solution to the long unsolved problem of attaining sufficiently high ethanol concentrations in the feed to the distillation column. Below 5 to 6% ethanol in the distillation column liquid raises the cost of recovering ethanol. I have discovered, however, that by using an ethanol-containing beer as a recycled diluent in the hydrolysis reaction step, in accordance with the process of this invention, the ethanol content in the feed to the distillation column can be maintained substantially above 6%. This results in producing ethanol at greatly lower cost and with less energy usage than otherwise possible.